In essentially every process in which a mixture is prepared for a particular purpose, the constituents of that mixture usually need to be present in particularly, proportioned amounts in order for the mixture to be effective for its intended use. In the aforementioned related patent applications, the underlying objective is to reduce the amount of organic solvent present in a coating composition by the use of supercritical fluid, particularly, carbon dioxide. Understandably, with this objective in mind, it is generally desirable to utilize as much supercritical fluid as possible while still retaining the ability to effectively spray the liquid mixture of coating composition and supercritical fluid and also obtain a desirable coating on the substrate. Accordingly, here too, it is particularly preferred that there be prescribed, proportionated amounts of supercritical fluid and of coating composition present in the liquid admixed coating formulation to be sprayed.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the coating composition. This practical upper limit is generally recognizable when the admixture containing coating composition and supercritical fluid breaks down from one phase into two fluid phases.
To better understand this phenomenon, reference is made to the phase diagram in FIG. 1 wherein the supercritical fluid is supercritical carbon dioxide fluid. In FIG. 1, the vertices of the triangular diagram represent the pure components of an admixed coating formulation which for the purpose of this discussion contains no water. Vertex A is solvent, vertex B is carbon dioxide and vertex C represent a polymeric material. It can be clearly seen in this Figure that the polymer and the solvent are completely miscible in all proportions, that the carbon dioxide and the solvent are likewise completely miscible in all proportions, but that the carbon dioxide and the polymer are not miscible in any proportion, and as such is a non-solvent for the polymer. The curved line BFC represents the phase boundary between one phase and two Phases. The point D represents a possible composition of a coating composition in which supercritical carbon dioxide has not been added. The point E represents a possible composition of an admixed coating formulation after admixture with supercritical carbon dioxide. The addition of supercritical carbon dioxide fluid has reduced the viscosity of the viscous coating composition to a range where it can be readily atomized by passing it through an orifice such as in an airless spray gun. After atomization, a majority of the carbon dioxide vaporizes, leaving substantially the composition of the original viscous coating composition. Upon contacting the substrate, the remaining liquid mixture of the polymer and solvent component(s) will flow, i.e., coalesce, to produce a uniform, smooth film on the substrate. The film forming pathway is illustrated in FIG. 1 by the line segments EE'D (atomization and decompression) and DC (coalescence and film formation).
The amount of supercritical fluid, such as supercritical carbon dioxide, that can be mixed with a coating composition is generally a function of the miscibility of the supercritical fluid with the coating composition as can best be visualized by referring to FIG. 1.
As can be seen from the phase diagram, particularly as shown by arrow 100, as more and more supercritical carbon dioxide is added to the coating formulation, the compositions of the liquid admixed coating mixture approaches the two-phase boundary represented by line BFC. If enough supercritical carbon dioxide is added, the two-phase region is reached and the composition correspondingly breaks down into two fluid phases. Sometimes, it may be desirable to admix an amount of supercritical fluid which is even beyond the two phase boundary. Generally, however, it is not preferable to go much beyond this two phase boundary for optimum spraying performance and/or coating formation.
In addition to avoiding the two-phase state of the supercritical fluid and the coating composition, proper proportionation is also desirable to provide optimum spraying conditions, such as, formation of desired admixed viscosity, formation of desired particle size, formation of desired sprayed fan shape, and the like.
Accordingly, in order to spray liquid admixed coating formulations containing supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intermittent or periodic on-demand basis, it is necessary to prepare such liquid admixed coating formulations in response to such spraying by accurately mixing a proportioned amount of the coating composition with the supercritical fluid. However, the compressibility of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure or temperature results in large changes in the density of the supercritical fluid.
The compressibility of the supercritical fluids causes the flow of these materials, through a conduit and/or pump, to fluctuate. As a result, when mixed with the coating composition, the proportion of supercritical fluid in the resulting admixed coating formulation also correspondingly fluctuates instead of being uniform and constant. Moreover, the compressibility of liquid carbon dioxide at ambient temperature is high enough to cause flow fluctuations to occur when using reciprocating pumps to pump and proportion the carbon dioxide with the coating composition to form the admixed coating formulation. This particularly occurs when the volume of liquid carbon dioxide in the flow path between the pump and the mixing point with the coating composition is too large. The fluctuation can be promoted or accentuated by any pressure variation that occurs during the reciprocating pump cycle.
In an embodiment discussed in a number of the aforementioned related patent applications, (U.S. application Ser. Nos. 218,896 and 218,910) an apparatus is disclosed for pumping and proportionating a non-compressible fluid, e.g., a coating composition, with a compressible fluid, e.g., liquid carbon dioxide, in order to prepare the ultimate mixture to be sprayed comprised of the coating composition and the carbon dioxide in its supercritical state. In that embodiment, volumetric proportionating of the coating composition stream and the liquid carbon dioxide stream is carried out by means of reciprocating pumps which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the coating composition which is slaved to another reciprocating pump which is used to pump the liquid carbon dioxide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The volume ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, since liquid carbon dioxide is relatively compressible at ambient temperature (the temperature at which it is typically stored in a pressurized container), such compressibility may undesirably cause fluctuations and oscillations of the amount of carbon dioxide that is present in the admixed coating formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of relatively non-compressible coating composition and relatively compressible liquid carbon dioxide. With the coating composition, pressure is immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is substantially compressible, a larger volume is needed to be displaced in order to generated the same pressure. Because mixing occurs when the flow of the coating composition and of the liquid carbon dioxide occurs in the mixing manifold at the same pressure, the flow rate of carbon dioxide lags behind the flow rate of the coating composition.
This fluctuation is accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor changing direction during its cycle. Thus, if the driving force declines, the pressure in the coating composition flow declines even more rapidly, due to its non-compressibility, than the pressure in the liquid carbon dioxide flow, due to its being compressible.
Accordingly, the pressures generated in both flows may be out of phase during the pumping cycle, such that the proportion of carbon dioxide in the mixture to be sprayed also varies. This fluctuation is made even more severe if cavitation also occurs in the carbon dioxide pump due to vapor formation as the pump fills.
While some of these fluctuations and problems have been suppressed by refrigerating the liquid carbon dioxide to low temperatures such as below 10.degree. C., and even below 0.degree. C., prior to its entering the reciprocating pump, a need still existed to avoid substantially all inaccuracies that may be present in the proportionation of the non-compressible coating composition and the compressible liquid carbon dioxide to form the desired admixture. Indeed, a need existed to provide a means to accurately proportion any compressible fluid with a non-compressible fluid.
That need was met in the aforementioned related patent application, U.S. patent application Ser. No. 413,517, filed Sep. 27, 1989, wherein apparatus and methods are disclosed for accurately and continuously providing a proportioned mixture comprised of non-compressible fluid and compressible fluid, relying particularly upon mass proportionation.
Generally, the apparatus disclosed in U.S. patent application Ser. No. 413,517 comprises:
a) means for supplying substantially compressible fluid;
b) means for measuring the mass flow rate of the substantially compressible fluid;
c) means for generating a signal in response to the measured mass flow rate of the substantially compressible fluid;
d) means for supplying substantially non-compressible fluid;
e) means for controlling the flow rate of the substantially non-compressible fluid responsive to the signal generated in (c);
f) means for forming a mixture of the measured compressible fluid and the controlled non-compressible fluid; and
g) means for circulation in which the mixture is introduced containing:
i) a heating means for heating the mixture to a temperature wherein the compressible fluid is in the supercritical state; PA1 ii) a positive displacement pump for circulating the mixture; PA1 iii) an accumulator; PA1 iv) a static mixer; PA1 v) a density meter; and PA1 vi) a spraying means.
The broadest method disclosed in that application for forming a mixture of a substantially compressible fluid and a substantially non-compressible fluid in a predetermined proportion includes:
a) supplying substantially compressible fluid;
b) measuring the mass flow rate of the substantially compressible fluid;
c) generating a signal in response to the measured mass flow rate of the substantially compressible fluid;
d) supplying substantially non-compressible fluid;
e) controlling the flow rate of the substantially non-compressible fluid responsive to the signal generated in (c);
f) forming a mixture of the measured compressible fluid and the controlled non-compressible fluid; and
g) introducing the mixture to a circulation loop in which the mixture is heated to a temperature above the critical temperature of the compressible fluid wherein the compressible fluid enters its supercritical state to form the mixture of non-compressible fluid and supercritical compressible fluid in a predetermined proportion.
As used in that application and as used herein the phrase "compressible fluid" is meant to include a material whose density is affected by a change in pressure to an extent of at least about 2 percent.
Specifically, as discussed in application Ser. No. 413,517, the mass flow rate of the compressible fluid is continuously and instantaneously measured. Regardless of what that flow rate is and whether or not it is fluctuating as a result of, for example, being pumped by a reciprocating pump or regardless of the state in which such compressible fluid is in, that mass flow rate information is fed to a signal processor on a continuous and instantaneous manner. Based on that received information, the signal processor in response to the amount of compressible fluid that has been measured, controls a metering device which controls the rate of flow of the non-compressible fluid. The non-compressible fluid is metered in a precise, predetermined proportion relative to the compressible fluid flow rate such that when the compressible and non-compressible fluids are subsequently mixed in the mixing manifold, they are present in the admixed coating formulation in the proper proportions.
By measuring the mass flow rate of the substantially compressible fluid, and then controlling the amount of non-compressible fluid in response thereto, the problems associated with phase changes of the compressible fluid, such as vaporization or condensation, are substantially eliminated. Any fluctuations or oscillations present in the flow of the compressible fluid are instantaneously measured and are compensated by controlling the amount of non-compressible fluid to provide the prescribed proportionation for the desired mixture. In contrast to past techniques, this embodiment involves the metering, i.e., controlling the flow rate, of only one fluid, namely, the non-compressible fluid. The flow rate of the compressible fluid is not controlled, but rather only measured, from which measurement the prescribed amount of non-compressible fluid is correspondingly adjusted to provide the desired proportionation. This allows for total flexibility of the system and provides for simple and effective means for producing the desired proportioned mixture of compressible and non-compressible fluids.
The apparatus and methods disclosed in application Ser. No. 413,517 are particularly effective and specifically focused for producing the desired proportionated mixture of compressible and non-compressible fluids on a relatively large scale, continuous basis. The suitability of the inventions disclosed in that Application is demonstrated most clearly by the Examples included in said Application wherein trouble-free operation was sustained as indicated by observation of the sight glass in the circulation loop of single-phase admixtures of coating composition and the compressible fluid, such as carbon dioxide, and by the subsequent successful spraying onto the substrate such that coatings with the desired properties being obtained.
In those Examples, the compressible fluid used was carbon dioxide, while the non- compressible fluid, the coating composition, consisted of one or more polymers and resins selected from a group including Rohm & Haas Acryloid.TM. AT-400 resin (an acrylic polymer dissolved in methyl amyl ketone solvent), American Cyanamid Cymel.TM. 323 resin (a melamine polymer dissolved in isobutanol solvent), Rohm & Haas Acryloid.TM. AT-954 resin (another acrylic polymer dissolved in methyl amyl ketone solvent), Rohm & Haas Acryloid.TM. B-66 resin (an acrylic polymer dissolved in methyl amyl ketone solvent), Spencer Kellog Aroplaz.TM. 6025-A6-80 resin (a polyester polymer dissolved in methyl PROPASOL.TM. acetate), Eastman Chemical cellulose acetate butyrate (a solid very high molecular weight polymer dissolvable in methyl amyl ketone solvent), Dow polystyrene polymer, Du Pont Centari.TM. Acrylic Enamel B8292A Medium Blue Metallic Auto Refinish Paint, a mixture of an alkyd resin, an acrylic resin, and a polyester resin dissolved in a mixed solvent containing xylene, methyl isobutyl ketone, and mineral spirits, and Eastman Chemical Cellulose Ester CAB-381-0.1.
As in application Ser. No. 413,517, in application Ser. No. 218,910, the effectiveness of the mixing point device, a commercially available Swagelok.TM. tubing tee fitting with a 5000-psi pressure rating which combines the coating composition and the supercritical fluid and conveys the admixture into a static mixer and then into the circular loop, was demonstrated by the Examples that were set forth wherein increasing the supercritical fluid (carbon dioxide) concentration to the solubility limit (a trace of fine bubbles appears in the circulation loop sight glass) and beyond did not result in the malfunctioning of the apparatus; although, in the latter case such operation in the two-phase region resulted in lower quality coatings. In these Examples, the Rohm & Haas Acryloid.TM. resins and American Cyanamid Cymel.TM. 323 resin were primarily used. Similarly, in other Examples of the application Ser. No. 218,910, dependable apparatus operation was experienced even when the admixed coating formulations contained suspended solids such as metallic flakes or white titanium dioxide pigment.
Furthermore, the effectiveness of the mixing manifold and the circulation loop apparatus and method was demonstrated by the Examples included in aforementioned U.S. patent application Ser. No. 327,274, wherein generally water was utilized to replace organic solvent present in a precursor composition including Cargill 7451 (a water-reducible tall oil fatty acid alkyd resin in butoxy ethanol solvent) and American Cyanamid Cymel.TM. 303 resin until the two-phase region was reached. In some cases substantial clouding of the solution developed, which indicated precipitation of polymer or resin beginning to occur, resulting in phase separation without affecting the spraying performance of those compositions nor the apparatus being utilized. When the resin was changed to Cargill 7203 (a water-reducible oil free polyester resin supplied in a 2-butanol:butoxy ethanol solvent system), similar results were obtained. In all of the cases where two-phases occurred, it was evidenced that the resulting phenomenon was the dispersement of the less viscous fluid as bubbles or particles in the more viscous fluid.
Thus, even under conditions in which two phases were obtained by design, it is clear from the foregoing discussion that the apparatus and methods disclosed in the embodiments of the aforementioned related applications were effectively supplying, feeding, measuring, proportioning, mixing, pressurizing, heating, and spraying an admixed coating formulation consisting of an admixture of a non-compressible coating composition comprised of one or more high solids resin or polymer selected from a substantial list comprised of acrylics, amino, polyesters, alkyds, a variety of organic solvents, including water in some instances; suspended solids such as metallic flakes and other pigments; and a compressible supercritical fluid, such as supercritical carbon dioxide, as a viscosity reducing diluent.
Unexpectedly, however, we experienced operating problems when a nitrocellulose lacquer based coating composition was used with the methods and apparatus disclosed in the preferred embodiments of the of aforementioned Applications. For reasons not fully understood with this coating composition, precipitation of solids occurred at the carbon dioxide injection and mixing point resulting in apparatus plugging; prior operation of the apparatus with the aforementioned acrylic and other polymer systems in said apparatus showed no evidence of such problems, even when some precipitation occurred.
After several runs with the nitrocellulose lacquer based coating composition, inspection of the apparatus revealed that the precipitate, in the form of a solid, partially to fully plugged the carbon dioxide 180.degree. mixing tee, followed by additional plugging through the accumulation of said solids in the downstream static mixer connected to the injection point device.
As used herein and as is conventionally used in the art, a "180.degree. mixing tee" is defined as a pipe or tubing tee in which two fluids are introduced opposing each other int he run of the tee with mixed flow exiting through the branch of the tee. On the other hand, a "90.degree. mixing tee" is defined as a pipe or tubing tee in which one of the fluids is introduced through the branch of the tee to mix with the primary flow in the run of the tee with the mixture exiting through the run of the tee.
Mixing tees are well known to those skilled in the art as a device to provide the mixing of two fluid streams provided pressure losses and mixing distances are not excessive. The purpose of said device is to quickly mix two streams. The results of studies of the functioning of such devices are available in the literature and, in general, show that the turbulence induced as two relatively non-viscous streams are brought together, either in opposition or at right angles, produces rapid mixing. By virtue of the said turbulence generated, mixing tees are normally considered to be turbulent mixers.
In our prior experience, when polymer or resin precipitated due to the planned or accidental addition of too much carbon dioxide, or perhaps due to the coating admixture infringing the two-phase region for one reason or another, or when finely divided solid particles were suspended in the admixture as part of the coating composition, we found no such gel formation nor experienced any observable plugging of the apparatus. Therefore, from a fluid dynamic standpoint, it appeared that the methods and apparatus of the aforementioned inventions heretofore employed comprised an effective system for achieving all operational objectives.
Without wishing to be bound by theory, we believe that the action associated with turbulence connected with the introduction and mixing of a polymer containing fluid and the supercritical carbon dioxide fluid within a mixing tee resuls in a condition wherein relatively large bubbles, plugs, or slugs, or even stratified flow patterns occur, wherein pure or relatively pure liquid supercritical carbon dioxide comes into contact at the liquid-liquid interface with the laminar layer of the polymer material admixture in contact with the tubing tee wall, such that the composition of said wall admixture, particularly at the liquid-liquid interface, is ever changing and eventually reaches a state wherein the polymer component precipitates once the two-phase region (as shown in FIG. 1 as bounded by line BFC) is reached and penetrated. This causes precipitation which results in the formation and growth of films and particles on the tee walls, eventually reaching a magnitude such that the tee becomes completely plugged. At the same time, other particles apparently slough off the growing solids mass and flow downstream causing plugging in other downstream devices of the apparatus. We also believe, and still not wishing to be bound by theory, that other aggregates of pure carbon dioxide in the mainstream may act upon aggregates of polymer solution causing localized precipitation and the development of particles in the flowing fluid.
Referring again to FIG. 1, when two phases occur, they are in equilibrium, with said equilibrium established by a series of tie lines (not shown in FIG. 1) spanning line BFC, and generally being somewhat reasonably parallel to the base line BC with the slope (increasing from side AB towards side AC of the triangle) of said tie lines changing as the infinite series of tie lines extending from the curve segment BF proceeding towards the critical point F. As can be seen in FIG. 1, if one were to extend line EE'D to intersect the FC segment of the equilibrium cure BFC, this Phase contains, and this is the usual case, a moderate amount of polymer and about a one-to-one ratio of solvent and non-solvent, which would vary, of course, from example to example, and the material of this composition is a swollen polymer, which can be a gel. On the other hand, the other phase, which would intersect segment BF of the equilibrium curve, nearer F than B in this case, is usually practically pure liquid, particularly when one branch of the binodal (curve segment BF in our case) is very close to the solvent-non-solvent side of the triangle, except, of course, near the critical point F. Generally, the solvent-non-solvent composition is usually very different in the two phases.
Because of its importance, a brief discussion of fluid mechanics principles is warranted. Without wishing to be bound by theory, in the single-phase flow of Newtonian fluids in circular conduits, flow velocity varies within the conduit due to the shear-stress distribution. For a uniform flow, the shear stress will be zero at the center and increase linearly to a maximum at the conduit wall. For laminar flow in a circular conduit, this shear-stress force distribution causes a parabolic velocity profile across the cross-section of the conduit with the maximum velocity at the center of the conduit. The layer of fluid near the solid surface of the conduit, the liquid-solid interface, wherein the change of velocity occurs due to the shear stress at said surface, is termed the "boundary layer" and for a laminar boundary layer its thickness is inversely proportional to the Reynolds number, based on the free-stream velocity. Therefore, because of the effect of the viscosity term upon the Reynolds number, the boundary layer thickness is larger for high viscosity fluids flowing at low velocities. Of course, at the conduit wall the velocity is zero, and material adjacent to said wall does not move, except by diffusional forces which are relatively small. Moreover, because of these conditions, with a high viscosity fluid such as in the present case, a relatively thick layer of relatively unmoving material exists at the conduit wall. When non-Newtonian fluids in laminar flow are considered, similar behavior is experienced; except that for pseudoplastic materials such as polymeric solutions or melts, the shear stress is no longer a linear function of the shear rate, but in general is a logarithmic function. However, the transition from laminar to turbulent flow occurs at approximately the same Reynolds number.
For the case of a turbulent boundary layer, however, the layer has three zones. The zone immediately adjacent to the surface of the conduit is a layer of fluid, which because of the damping effect of the conduit surface, remains relatively smooth although most of the flow in the boundary layer is turbulent. This very thin layer is termed the "viscous sublayer" and its thickness is a direct function of viscosity and an inverse function of the shear velocity at the conduit surface. For high velocity flow, therefore, the viscous sublayer thickness will be small. The flow zones outside of this sublayer are turbulent wherein the mixing action of turbulence causes small fluid masses to be swept back and forth in a direction normal to the mean flow direction. Consequently, primarily a momentum-exchange phenomenon is effected as fluid is swept from one zone to another.
For purposes of the present discussion, "turbulent flow" is characterized by mixing action throughout the flow field that is caused by eddies of varying size within said flow field. "Laminar flow", on the other hand, is lacking the strong mixing phenomena and eddies common to turbulent flow. Therefore, the flow has a very smooth aspect. For the case of laminar flow in a straight circular conduit, as previously discussed, the velocity distribution is parabolic, and at any given distance from the conduit wall the velocity will be constant with respect to time. In the turbulent-flow case, two effects are apparent. First, because the flow is thoroughly mixed due to the eddies, the velocity distribution is more uniform than in the laminar-flow case. This occurs because the turbulent mixing process transports the low-velocity fluid near the wall towards the center, and the higher-velocity fluid in the central region is transported toward the wall. The second effect of turbulence is to continuously add fluctuating components of velocity to the flow field. At any instance of time the distribution of the velocity component in the axial direction of flow is, therefore, irregular. Accordingly, the velocity varies with time, and the flow is termed unsteady. However, for the average mean velocity with respect to time at a given point taken over a relatively long time period, the velocity is practically constant and the flow is termed steady.
Further complicating the understanding of the fluid dynamics occurring as carbon dioxide and coating material liquids are merging, is the existence of two-phase flow. Very little experimental work has been done, as reported in the literature, on liquid-liquid, two-component isothermal flow in circular conduit systems. Most of the known work reported concerns liquid-gas systems, liquid-solids systems, and solids-gas systems. What work that has been done on the liquid-liquid system has concentrated around the understanding of the two-component flow of the immiscible system of oil and water. For a nearly equal density oil-water system, the flow patterns are very similar to those described for liquid-gas flow, i.e., bubble, plug, stratified, wavy, slug, annular, and spray flow. For bubble and slug flow, bubbles or slugs of the more viscous liquid occur in the less viscous fluid phase. For the stratified flow regime, the less viscous fluid phase becomes a concentric annular flow around a core of the more viscous fluid. When the velocities of the two fluids are relatively low, less than about 2 feet per second, apparently only bubble, plug or stratified flow patterns occur; wherein the regime which prevails is dependent upon the ratio of their respective velocities. When the velocity ratio of the more viscous fluid to the lesser viscous fluid is about 2, with a low flow velocity of about 0.3 feet per second for the lesser viscous fluid, the flow pattern is in the stratified regime. When the velocity ratio of the more viscous fluid to the lesser viscous fluid is reduced to about 0.15 feet per second, the flow regime undergoes a transition to the plug or bubble regime.
Furthermore, when two phases are present either can be laminar, transitional, or turbulent. Accordingly, in stratified flow, for example, the lower viscosity fluid may be in any of these three states while the more viscous fluid may be in the laminar state. In the bubble flow pattern in horizontal circular conduits, it has been observed that the bubbles of the lower density fluid travel along the top of the higher density fluid, while in vertical conduits the bubbles of the lower density fluid travel inside the higher density fluid.
For the case of the injection of the less viscous fluid around the periphery of a horizontal circular conduit when using fluids of equal density, it was observed that the more viscous fluid flowed inside of the less viscous fluid. Of course, this represents the case of an annular flow pattern, but the flow could also be in the stratified pattern, or intermediate between them depending upon the velocities and volumes of flow of the two fluids. With a stratified flow pattern under conditions of laminar flow in a horizontal circular conduit, it would be expected that the less viscous fluid would flow above the more viscous fluid, with the location of the interface between the two fluid dependent upon their volumetric ratio. When considering fluids of unlike density in stratified flow, the lighter fluid would tend to flow above the more dense fluid, and when the lighter fluid is also the less viscous fluid enhancement of this effect would be expected to occur.
Regardless of whether the flowing fluid is in the laminar or turbulent flow regime, a laminar or viscous layer of fluid resides at the conduit wall surface wherein turbulence is absent and there is no mixing normal to the direction of flow within the conduit. Of course, some movement of components normal to the direction of flow is expected because of diffusion. In the present invention, where the flow regime is in the laminar region, we are operating in the state described as the laminar boundary layer, which results in our experiencing the thicker layer of whatever fluid may be present therein.
Applying these fluid dynamic criteria with the assumption that they apply in kind to the partially miscible complex fluid liquid-liquid system of a nitrocellulose lacquer coating composition and supercritical carbon dioxide mixture of the present instance, it is believed that the point within the present apparatus with the greatest susceptibility for precipitation exists within the 180.degree. mixing tee run, with the tee positioned with the run in the horizontal position, wherein the coating composition and the carbon dioxide flows meet after the injection of carbon dioxide in the turbulent flow regime into the run of the mixing tee opposite to the injection of coating material fluid in the laminar flow regime into the other end of the run of said tee with the combined flow exiting from the branch of the tee in the laminar flow regime; and, next within the branch of said tee into which both streams flow first into and then out of.
Although the actual flow pattern existing within the horizontal 180.degree. mixing tee is unknown, we believe precipitation of polymer occurs by the effect of one or more of the bubble, plug, stratified, or annular flow patterns established when the turbulent flowing non-solvent supercritical carbon dioxide liquid is injected and then comes into contact with the coating composition present in the near-zero-velocity relatively thick boundary layer at the walls of the run and branch of the mixing tee. Within said region supersaturation is reached rapidly followed by the eventual dissolution of polymer, thereby resulting in the growth of polymer upon the wall surface of the mixing tee.
Particularly, we would expect such results with the run of the tee and associated tubing in a horizontal position, wherein the less dense, less viscous carbon dioxide fluid would be expected to be in an outer annulus around a core of coating material fluid, if the flow pattern is annular, and precipitation of polymer occurring either: (1) upon the walls, or (2) at the interface between the two fluids, or in the peripheral region, from which it could easily become attracted to the wall of the tee and growing thereon resulting in its plugging or the sloughing off of solid material, which then can plug downstream apparatus. Moreover, all of the above mechanisms could be present and contributing to such plugging. Similar phenomenon would occur should the flow pattern be bubble, plug, or stratified, wherein the less dense less viscous carbon dioxide fluid would flow above the more dense more viscous coating material fluid and come into contact with coating material on the wall. We would expect that this type of flow pattern would exist regardless whether the injected carbon dioxide fluid is in laminar or turbulent flow.
In one experiment that we conducted, a static mixer was incorporated into the apparatus between the coating composition 90.degree. mixing tee and the supercritical carbon dioxide 90.degree. mixing tee. We observed partial blocking of the circulation loop as indicated by about a 20 percent drop in the circulation loop pressure accompanied by about a 40 to 50 percent drop in supercritical carbon dioxide feed rate, and surging would occur as plugs broke loose causing about a 500 percent increase in the carbon dioxide feed rate, which rate was 200 percent greater than the normal feed rate. Where operation of the apparatus was allowed to continue, we observed eventual stalling of the loop pump. Inspection of the disassembled equipment after shutdown showed partial blockage of the static mixer that was located downstream of the carbon dioxide 90.degree. mixing tee, and the 3/8 inch tubing mixing tee used was found to be almost completely plugged with very hard solid polymer. It is apparent, therefore, that providing intensified mixing in this manner at this point in the apparatus was ineffective.
With this same apparatus, in cases where spraying was discontinued, which constitutes a normal condition between actual spraying of the admixed coating formulation with continual flowing of the admixed coating formulation within the circulation loop without addition of either coating composition nor supercritical carbon dioxide fluids into the loop, the problem would slowly correct itself, which indicated that the precipitated polymer gel was eventually going back into solution because the composition in the circulation loop was such that at equilibrium a single-phase system should exist. However, the time required for this event to occur was not short enough to resolve the problem in a practical manner.
In another experiment that we conducted, the apparatus was modified to enhance turbulence within the carbon dioxide 90.degree. mixing tee by installing a Circle Seal check with the exposed seat screwed into a 3/8 inch pipe tee used as the carbon dioxide injection point. We observed an increase in the rate of plugging to an extent that it occurred immediately after initiating the carbon dioxide feed causing plugging such that the carbon dioxide feed rate never reached the design rate. Inspection of the apparatus after shutdown showed the carbon dioxide injection point (the 3/8 inch pipe tee run) to be nearly blocked solid with a hard solid polymer. Although, based on theoretical expectations, the same flow pattern should exist regardless of the flow regime of the carbon dioxide, either laminar or turbulent, or in transition between the two, apparently turbulence produced enough eddies or other phenomenon such that plugging was intensified.
Confirmation of the undesirability of having turbulence at the carbon dioxide injection point was obtained in Yet another experiment which we conducted in which the apparatus was modified in an attempt to minimize eddies by the close coupling of apparatus components in the polymer and carbon dioxide injection points. The results in this case showed a reduction in the plugging over the aforementioned experiments, probably because only a thin film of polymer was being formed at the carbon dioxide point of entry at the injection point. Although, we observed repeated plugging, it also blew free. The undesirable less severe surges in the carbon dioxide flow rate that occurred, dropping by about 10 percent and then surging upward by about 180 percent to about 150 percent of normal flow rates, however, were still too excessive for the effective operation of the spray coating apparatus.
From the preceding discussion, it is clear that at the point of mixing the supercritical carbon dioxide non-solvent liquid and the polymer solution fluid, precipitation of polymer could and does occur due to effects governed by principles of fluid mechanics and by principles associated with the thermodynamics of phase equilibrium.
We believe that the precipitation phenomena is related to the structure of the polymers or resins utilized and how such structure influences the redissolving of and other phenomena associated with the Precipitated polymer or resin.
Without wishing to be bound by theory, the structure of importance relates to the nature of the molecular order of long-chain molecules, such as polymers or resins. Many polymers, when solidifying from their melts, assume the properties of glasses or supercooled liquids. That is, they do not exhibit signs of anisotropy, their x-ray diagram looks like that of a liquid, and they soften gradually with increasing temperature and harden reversibly upon cooling. Seemingly, the macromolecules are randomly arranged in space. Many linear polymers and copolymers occur primarily in this state, they exhibit the properties of structureless resinous plastics that become brittle at low temperatures, and at elevated temperatures either soften gradually to form viscous melts or harden to form insoluble and infusible thermosetting resins.
Many other linear polymers, however, display a different behavior. Specifically, if cooled from a melt or precipitated from solution, the macromolecules of these polymers show a definite inclination to establish arrays of lateral order wherein the axes of the individual chain molecules are parallel. In a sense, as one passes in a direction perpendicular to the chain axes, a certain amount of regularity exists. These laterally ordered areas are termed "crystallites" or "micelles," although they do not have all of the properties historically associated with these terms. It appears that crystallization of macromolecules is generally favored when the chains possess a regular structure. On the other hand, however, macromolecules with irregular chain structure, particularly random copolymers, favor the formation of amorphous polymers. Although, very few, if any, are completely amorphous or crystalline; almost all polymeric substances lie between these two extremes of order and disorder. The ratio of crystalline to amorphous content has a great influence upon the properties of the polymers.
The crystallites of a polymer can be different in size, shape, and may be oriented, and they are intimately connected with the amorphous matrix because the same macromolecule may pass through several crystalline and disordered areas. In the aggregate, polymer chains may go through more than one crystalline and amorphous region. Therefore, depending upon their molecular order, polymers have crystalline-amorphous character which depends on many factors, not the least of which are the monomer(s) from which they are derived, and the molecular weight of the polymer chain produced.
The crystalline-amorphous character of the polymer can have a significant effect upon its properties. With respect to solubility, for example, linear amorphous polymers dissolve more readily in a wide range of organic solvents than do predominately crystalline polymers, which can be dissolved only under limited conditions in a few specific solvents, or than those for that matter with three-dimension spatial configuration, which are most often nearly insoluble. However, crystalline polymers can be dissolved readily above their melting point, where sufficient thermal energy has already been supplied to separate the polymer molecules from their tight crystalline structure. Flexible linear macromolecules such as starch are more soluble than rigid molecules such as cellulose, for example, i.e., very rigid polymers show little solubility. Crystallinity decreases the solubility of polymers markedly, mainly because the process of solution consists of overcoming the heat and entropy factors identified with crystallization as well as those of the intermolecular interactions in the amorphous regions.
As aforementioned, when a certain concentration is reached as non-solvent is added to a polymer solution, the homogeneous liquid separates into a mobile, liquid phase and a swollen polymer or gel phase. When this gel phase consists of linear polymer with predominately amorphous character, such gel is easily penetrated by solvent which very readily can result in the precipitated swollen polymer redissolving. On the other hand, when the gel phase consists of linear polymer with predominately crystalline character, solvent does not readily penetrate that portion of the gel which contains the crystallites or micelles and redissolving becomes most difficult, particularly if the crystallites have diffused to become growing spherulites. Moreover, in the latter type of polymer, solubility is usually confined to the amorphous regions, and although these regions would be solubilized, the crystalline portion may well not be, as crystallization is not always reversible and the best which may be expected is the swelling of the gelatinous mass.
Accordingly, we have observed that once the polymer is precipitated, its ability to be redissolved, or go back into solution in the presence of solvent, is influenced markedly by the amorphous-crystalline character of the polymer.
Using this rational, the polymers specified in the coating compositions utilized in the aforementioned Examples of the related Applications and Experiments which we conducted were scrutinized. Based upon the best data available in the literature, it is believed that of the polymers and resins used, only the nitrocellulose resin has considerable crystalline character. In fact, cellulose, the nitrocellulose precursor, is a regular crystalline array of parallel helices, and the hydroxyl groups of the monomer unit produce a buildup of hydrogen bonding which is strong, high-melting, and insoluble. The other polymers, from the group comprising acrylics, amino, polyesters, alkyds, etc., are amorphous, with very little, if any, crystalline character.
We therefore believe, again not wishing to be bound by theory, that in the mixing tee used with the apparatus and methods disclosed in the aforementioned related patent applications, when bubbles, plugs, slugs or stratified flow of carbon dioxide, a non-solvent for the polymer, reaches the polymer solution present in the laminar boundary layer, it causes precipitation in the form of a swollen gel that apparently forms a film and further grows into significant deposits of polymer. When the polymer so deposited has more crystalline character, such as we observed with the nitrocellulose, we believe that it was not being readily dissolved as the deposit's surface is swept by components of the flowing fluid high in solvent concentration such that dissolution of polymer in the said deposit would otherwise be possible, because the said composition can accommodate the deposited polymer without penetrating the two-phase boundary. Such being the case, the deposit continues to grow until it completely plugs the injection and mixing tee and/or downstream devices when some of the deposit dissociates and is not dissolved prior to reaching said devices. Since the liquid flow is laminar, mixing normal to the flow axis, except by diffusion, does not occur, which could disperse somewhat better bubbles, plugs, etc., of carbon dioxide present, and in effect aid in dissolving precipitated and deposited polymer.
On the other hand, when the polymer in the coating composition is amorphous, or amorphous with only little crystalline character, it is believed that the same phenomenon occurs. That is, as before, polymer is precipitated and deposited in the mixing tee as a film or as other similar deposits because of bubbles, plugs, etc., of carbon dioxide, but in this case when the polymer is amorphous, the film is readily penetrated by solvent and coating composition sweeping the surface of the film or deposits and dissolution occurs. Moreover, an equilibrium is probably established resulting in a residual thin film that does not continue to develop into a substantial deposit that could cause plugging. Likewise, should any precipitate disengage as discrete particles from the wall, or for that matter any particles be developed in the mainstream of the flow itself by bubbles, plugs, etc., of carbon dioxide, such particles should readily be dissolved by the same mechanism.
Further, if such particles persisted, it is hypothesized they would exist in a matrix such that solvent would surround the swollen polymer thereby minimizing the probability of agglomeration with the formation of gels and/or deposition within the spray apparatus. In effect, such particles would behave as discrete particles much like other solid insolubles, such as pigments, metallic flakes, and the like.
Such a hypotheses is reinforced by what was experienced in aforementioned U.S. patent application Ser. No. 218,910, and Ser. No. 327,274, with coating compositions comprised of amorphous polymers and resins, wherein the system was purposely projected into the two-phase region, signified by solid particles of polymer precipitated from solution, without any observable plugging of the apparatus, nor substantial change in the quality of the coating sprayed onto the substrate.
Clearly, what is needed is a simple method and apparatus to introduce a non-solvent, such as supercritical carbon dioxide, into a fluid containing a dissolved solid, such as a polymer or resin, for example. The method and apparatus should be such so as to prevent the deposition of solids and the possible consequential plugging at the mixing point, and in other downstream apparatus, from the saturation induced precipitation, for example, of polymer(s) and resin(s) in coating compositions and admixed coating formulations by supercritical carbon dioxide fluid acting as a precipitant, as the coating composition fluid and the supercritical fluid liquid are introduced into the apparatus and are mixed and commingled therein.
In particular, methods and apparatus are needed wherein saturation of highly crystalline character polymer(s) and resins(s), does not occur through the contacting of said material by bubbles, plugs, slugs of the non-solvent, such as supercritical fluids, such as carbon dioxide, or even from stratified or annular flow patterns of the same, thereby avoiding the aforementioned precipitation and adherence of said solids within the apparatus and, accordingly, preventing eventual plugging in the apparatus.
The problems that we have recognized cannot be practically and economically solved using conventionally available devices.